1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording head for use in thermally-assisted magnetic recording to write data on a recording medium with the coercivity thereof lowered by irradiating the recording medium with near-field light.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head unit including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head unit including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider configured to slightly fly above the surface of a recording medium. The slider has a medium facing surface configured to face the recording medium. The medium facing surface has an air inflow end (a leading end) and an air outflow end (a trailing end).
Here, the side of the positions closer to the leading end relative to a reference position will be defined as the leading side, and the side of the positions closer to the trailing end relative to the reference position will be defined as the trailing side. The leading side is the rear side in the direction of travel of the recording medium relative to the slider. The trailing side is the front side in the direction of travel of the recording medium relative to the slider.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near the medium facing surface of the slider.
U.S. Patent Application Publication No. 2011/0058272 A1 and U.S. Pat. No. 8,614,932 B1 each disclose a technology in which the surface of the core of the waveguide and the surface of the plasmon generator are arranged to face each other with a gap therebetween, so that evanescent light that occurs from the surface of the core based on the light propagating through the core is used to excite surface plasmons on the plasmon generator to generate near-field light based on the excited surface plasmons.
A thermally-assisted magnetic recording head that employs a plasmon generator as a source of generation of near-field light is configured so that the write head unit includes a coil, a main pole and the plasmon generator. The coil is configured to produce a magnetic field corresponding to data to be written on a recording medium. The main pole has an end face located in the medium facing surface. The main pole is configured to allow a magnetic flux corresponding to the magnetic field produced by the coil to pass, and configured to produce a write magnetic field from the aforementioned end face. The plasmon generator includes a near-field light generating part located in the medium facing surface. It is required of the thermally-assisted magnetic recording head that the end face of the main pole and the near-field light generating part of the plasmon generator be located in close proximity to each other.
To increase the linear recording density of magnetic recording devices, it is effective to use a perpendicular magnetic recording system in which the direction of magnetization of signals to be written on tracks of a recording medium is perpendicular to the plane of the recording medium, and it is also effective to increase, on the tracks, the gradient of the change in write magnetic field intensity with respect to the change in position along the direction in which the tracks extend or the direction along the tracks (this gradient will hereinafter be referred to as the write field intensity gradient). The same holds true for a magnetic recording device that employs thermally-assisted magnetic recording.
In order to increase the write field intensity gradient in a magnetic head of the perpendicular magnetic recording system, it is effective to provide a shield that has an end face located in the medium facing surface at a position near the end face of the main pole. U.S. Patent Application Publication No. 2011/0058272 A1 and U.S. Pat. No. 8,614,392 B1 each disclose a technology for increasing the write field intensity gradient by providing such a shield on the leading side of the main pole.
A magnetic head including a shield is typically provided with a return path section for connecting the shield to a portion of the main pole located away from the medium facing surface. One or more spaces are defined between the return path section and the main pole. The coil is provided to pass through the one or more spaces.
Thermally-assisted magnetic recording heads are being often used in high-end, large-capacity magnetic disk drives typified by those for cloud computing business. Highly reliable thermally-assisted magnetic recording heads applicable to high-end, large-capacity magnetic disk drives are thus in demand.
However, thermally-assisted magnetic recording heads suffer from the problem that heat generated by the plasmon generator causes the plasmon generator to shrink and become distant from the medium facing surface, and causes corrosion of the main pole, thus reducing the life of the thermally-assisted magnetic recording head.
One of solutions to this problem is to reduce the amount of heat to be generated by the plasmon generator by reducing the laser light power, and at the same time, improve the signal-to-noise ratio (hereinafter referred to as S/N) of the write head unit in order to make up for the performance reduction of the write head unit caused by the reduction in the amount of heat generated by the plasmon generator. A well-known method for improving S/N of the write head unit is to increase the number of coil turns in the write head unit. Conventionally, however, an increase in the number of coil turns has resulted in an increase in the length of the magnetic path including the main pole and the return path section, thus giving rise to problems such as sluggish rise of the write current flowing through the coil and a reduction in the write current in a high frequency region.